Physical, continuous-tone images such as photographs and paintings can have an unlimited range of colors. In contrast, the range of available colors available for a digital image is limited. When visually displayed, a digital image consists of a grid of pixels selected and arranged to reveal any combination of text and/or graphics. Where two hundred fifty-six or more colors are available for each pixel, the difference between one shade and the next in adjacent pixels becomes difficult if not impossible for the human eye to perceive. A digital image can be one of or incorporated within any number of formats. Format examples include PDL (page description format), PDF (Portable Document Format), and bitmap, TIFF (Tagged Image File Format), and JPEG (Joint Photographic Experts Group).
A digital image can be manipulated by performing one or more operations on its pixels. As examples, a digital image can be scaled to increase or decrease its number of pixels, its pixels can be sharpened, and its pixels can be smoothed. After scaling a digital image such as a scanned color photograph, the transition between adjacent pixels becomes visibly non-continuous. This degradation can cause the image to appear jagged. As a remedy, the scaled but degraded image can be smoothed to create more continuous transitions between adjacent pixels.
However, where the digital image includes text, smoothing can cause a degradation in the perceived print quality. A fine example is black text on a white background. The non-continuous transition allows the text to stand out to the human eye as well as to programming capable of optical character recognition. Smoothing can cause the text pixels to blend into the background substantially reducing a visible transition. Consequently, after enlarging a digital image containing text, smoothing the text pixels is often not desirable.